1. Field of the Invention
The invention relates to an iron removal procedure in the supersaturated sodium aluminate liquors obtained from alkaline attack according to the Bayer procedure of bauxite containing alumina monohydrate.
2. Description of the Background
The Bayer procedure widely described in the specialized literature constitutes the essential production technique used in the production of alumina intended to be transformed into aluminum by ignited electrolysis or to be used in the hydrate state, as transition alumina, charred alumina, sintered or melted alumina, in the numerous applications concerning the technical field of alumina.
According to that procedure, the bauxite is treated under heat with an aqueous solution of sodium hydroxide of suitable concentration or attacking liquor, thus making the alumina soluble and obtaining a sodium aluminate supersaturated solution.
After separation of the solid phase containing the untouched ore residue (red mud), the sodium aluminate supersaturated solution is seeded with particles of alumina trihydrate in order to induce the precipitation of alumina as alumina trihydrate. The sodium aluminate liquor which is now poor in alumina and which is called a decomposed liquor, is recycled in the attacking liquor step after having been concentrated and eventually recharged with sodium hydroxide to make the new attacking liquor.
It is known to one of ordinary skill in the art that the treatment conditions have to be changed according to the degree of hydration and to the crystallographic structure of the alumina as well as the nature and the content of the impurities found in the bauxite. That is why bauxite containing alumina in the monohydrate state (bohemite, diaspore) is treated at a temperature higher than 200.degree. C. and generally between 220.degree. C. and 300.degree. C., with extraction yields of soluble alumina exceeding 95%. The use of an attacking solution with high concentration of sodium hydroxide, usually between 130 and 180 g of Na.sub.2 O/liter, allows the soluble alumina to dissolve rapidly in a low volume of liquor. This translates into a high productivity of the supersaturated liquor, that is to say a production capacity after precipitation with seeds and cooling, of at least 80 kg of alumina per m.sup.3 of supersaturated liquor. This productivity is usually measured by the product of the sodium hydroxide concentration, C, expressed in g of Na.sub.2 O/liter with the variation .DELTA.Rp of the ratio Rp of soluble Al.sub.2 O.sub.3 concentration in g/liter/Na.sub.2 O g/liter between the beginning of the decomposition (with preferably 1&lt;Rp&lt;1.25) and the end of the decomposition (with preferably 0.5&lt;Rp&lt;0.7).
Consequently, those relatively harsh conditions make certain impurities soluble, and, in particular, the iron present in the bauxite in oxidized forms such as hematite, goethite and even magnetite and ilmenite. This iron can be made partially soluble following unexplained pathways by forming soluble complexes such as the ferrate ion Fe (OH).sub.4 - but it can also be precipitated in the colloidal state as very fine hydroxide particles (0.1 to 3 micrometers) which cannot be stopped by filtration, thus polluting the supersaturated sodium alumina liquor as well as the alumina trihydrate precipitated during the decomposition of the liquor.
Among the known procedure to eliminate the iron from the Bayer liquors, few of them bring a satisfactory solution to the problem of contamination of the alumina by the iron contained in the bauxite, especially if one wants to achieve an iron content in Al.sub.2 O.sub.3 in the alumina trihydrate state of less than 0.01%, considered today as the maximal admissible content for most of the alumina applications and specially in the field of technical alumina. On the practical side this implies for one of ordinary skill in the art that the iron content in the supersaturated aluminate liquor should be less than 10 mg Fe per liter.
A first category of procedures uses filtration techniques of the supersaturated liquor before the more selective decomposition but these procedures are almost impossible to implement in industry, such as filtration on sand or bauxite beds according to U.S. Pat. No. 3,792,542 and U.S. Pat. Nos. 3,728,432 or on special paper filter such as in U.S. Pat. No. 4,446,177.
Other methods more easily implemented in industry use additives which facilitate the agglomeration of colloidal iron and silica hydroxide particles and then the decantation and filtration of the red mud. Among these additives there are the synthetic flocculating agents such as the anionic polyelectrolytes with high molecular weights made generally of polyacrylates and introduced preferably at the beginning of the decantation step (U.S. Pat. No. 3,390,959, U.S. Pat. No. 3,681,012 or U.S. Pat. No. 3,975,396).
One has to talk also about mineral additives such as lime. This one is often used as an additive before the alkaline attack of the bauxite to make soluble the small quantities of sodium and alumina combined originally in the bauxite as salts or complex insoluble oxides with impurities such as silica and iron, titanium or vanadium oxides.
The lime can also be introduced before the decantation of the suspension obtained from the alkaline attack at high temperature of the bauxite and cooling to near 100.degree. C. by reducing the pressure until atmospheric pressure is reached. This is done to speed up the decantation of the red mud and to facilitate the separation of certain impurities such as the iron. That is what U.S. Pat. No. 3,796,789 recommends in order to lower to less than 10 mg per liter the iron content of the sodium aluminate liquors, especially those coming from the lixiviation of the sintered mixture of sodium carbonate and insoluble residues coming from a first alkaline attack of the bauxite (sinter process). This document doesn't specify, however, the "de-ironization" performances that can be achieved in a liquor directly obtained from the alkaline attack of the bauxite which is still the major problem to solve.
During those different experiments, the applicant can realize that the methods using those different additives sometimes allows the iron content of the supersaturated liquor from the alkaline attack of the bauxite to be lowered appreciably, but in any case, the method doesn't guarantee a content lower than 10 mg of iron per liter, whatever the origin of the bauxite is.
This result is attained however with procedures recommending at least one specific and complementary elimination step of the impurities present as colloidal hydroxide or the more or less soluble meta-stable complexes, such as iron. Thus, U.S. Pat. No. 3,607,140 and EP 0231715 (equivalent to U.S. Pat. No. 4,732,742) realize before decomposition a pre-decomposition of the supersaturated liquor in the presence of a very small quantity of seeds during which the impurities, and especially the iron, coprecipitate with the fraction of alumina trihydrate which is sacrificed as an impurity trap. Although efficient, those procedures increase production costs by slightly decreasing the productivity of the liquor and by increasing the time spent in the decomposition process lines.
For one of ordinary skill in the art, the achievement of a procedure eliminating the iron in the supersaturated sodium aluminate liquor before decomposition to a content of less than 10 mg of Fe per liter without changing the economical conditions of production, and notably productivity of the liquor and the yield of the extraction of soluble alumina, remains a priority.